Green, yellow and black? Very odd combination, seems to have been wired in a non-standard way. What you could do is switch off all the power and open up the switch and see if you can work out from that which wire is live/neutral/earth.

There is the school boy way to wire two way switches and the electricians way to wire two way switches. With the school boy method you need a connector in the switch but the way electricians wire them this is not required.

In the old days only two wires were run from one two way switch to other and the live was borrowed from another switch. However this produced mains hum and was discontinued and now three wires plus earth are used. However where the old system was used then upper and lower floors were split it also produced what is know as borrowed neutral which is very dangerous.

As a result I am wary about giving instructions as it is so easy to miss something in the telling and end up with something dangerous.

The are plans in the projects section plus many more on the net. But all line wires should be red in old houses and brown in new houses and where yellow or blue are used for line wires then they should have a sleeve on them. So there is no colour code to help you.
Note:- L = Line not Live as both the phase wire (Line) and Neutral wire are both considered as "Live".
Also often Blue is used for switched Line so many mistakes are made assuming blue is neutral. In theory blue wires or any other colour for that matter should have correct colour sleeve (Red or Brown) if used as a Line wire but often these sleeves are missing.